1. Field of the Invention
The invention, in general, relates to electromagnetic wave receivers and in particular to a receiver that is capable of receiving waves over a broad bandwidth and at the same time having good sensitivity.
2. Description of the Prior Art
Electromagnetic wave receivers in the broadcast band range, UHF (ultrahigh frequency) range, and VHF (very high frequency) range for receiving either FM (frequency modulated) or AM (amplitude modulated) signals are common. The disclosure of the present invention, in order to be specific, will be in terms of a UHF FM receiver, however, it should be understood that the invention is not limited to such receivers. As is conventional in the art, the electromagnetic waves shall be, in certain instances, referred to as rf (radio frequency) waves, however, it should be understood that this does not limit the invention to radio receivers.
FM receivers receive and demodulate an electromagnetic wave comprised of a fundamental carrier frequency modulated by a message signal. At the higher UHF frequencies, say for example, 320 megahertz, it is useful to employ a Surface Acoustic Wave Resonator (SAWR) in the transmitter because such resonators operate efficiently at high frequencies. However, such resonators also are characterized by wide manufacturing tolerances of the fundamental frequency, typically about .+-.200K hertz and provide modulation frequency changes (deviation) of only about 50-80K hertz. Reception of such signals is difficult or impossible with conventional receivers.
The principal reason for the inability of prior art receivers to receive such signals lies in the design of the detector (sometimes called the demodulator), which is that portion of the receiver which separates the message signal from the carrier wave. Conventional detectors are designed with a relatively high Q value. Q is a dimensionless variable sometimes referred to as the Quality Ratio. It may be defined either in terms of bandwidth, in which case it is given as Qbw=Fo/.DELTA.F where Fo is the fundamental frequency and .DELTA.F is the modulation deviation. Or it may be equivalently defined in terms of reactance and resistance as Qzr=Z/R where Z is equal to the difference between the inductive reactance and capacitive reactance and R is the DC or skin effect resistance. Prior to the present invention, it has been thought by those skilled in the art that the Q value of a detector circuit must be kept as high as possible to obtain good sensitivity. This is because a high Q value results in a high signal to noise ratio, which was thought to be necessary for good sensitivity.
As will be seen, the invention in one aspect also relates to the squelch circuitry of the receiver. In conventional FM squelch circuits, the detector output is provided with sufficient capacitive filtering to reduce the noise below a predetermined level which is less than the detector message signal level when a carrier is present. When no carrier is present, the squelch circuit blocks the output signal from the receiver output. When the higher voltage carrier message signal is detected, the output circuit is tripped to gate the detector signal to the output.
In order to obtain the signal to noise ratios required in the prior art detector circuits, it was found necessary to permit the filter capacitors in the circuits to discharge essentially completely. Such discharges generally require times of the order of 1/2 second. Thus prior art detectors have had response times of the order of 1/2 second or higher.